Method for producing foundry shapes

ABSTRACT

A method for producing silica sand-based foundry shapes useful informing metal castings and for reducing veining defects in sand-based foundry shapes by providing a foundry sand, adding an anti-veining composition that comprises bentonite to the foundry sand to form a mineral composition, then adding a foundry resin to the mineral composition to form a sand-based foundry composition and shaping the sand-based foundry composition to form a desired pattern.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. ProvisionalApplication Serial No. 60/332,679, filed Nov. 14, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method for producing foundryshapes and, more specifically, to a method of reducing veining defectsin sand-based foundry shapes by adding an anti-veining compoundcomprising bentonite.

[0003] Sand casting is a process used in the foundry industry to producemetal parts. In sand casting, disposable foundry shapes, such as coresand molds, are made by forming a sand-based foundry composition into thedesired shape and curing the composition. One or more binders mixed withthe silica sand are required to maintain the sand in a predeterminedshape. Commonly employed binders include inorganic binders such as clayand foundry resins such as phenolic resin binders. There are two basistypes of binder systems used in the foundry industry. Green sands areproduced by binding silica sand with clay, coal dust, and water.Chemically bonded sands use a variety of organic and inorganic resinbinders.

[0004] Green sand molding is the production of molded metal objects fromtempered molding sand and is widely used to cast ferrous as well asnon-ferrous metal castings. Green sand molding is economical and permitsboth quality and quantity production, particularly for smaller castings.Green sand is defined as a water tempered molding sand mixture withplasticity. A green sand mold used for casting steel usually consists ofsilica sand, and a binding agent mulled together with tempered water.Other useful foundry sands include chromite, zircon and olivine sands.

[0005] Chemically bonded sands refer to sand-based foundry compositionscomprising sand and a binding amount of a polymerizable or curablebinder. The binder permits the foundry composition to be molded orshaped into the desired form and thereafter cured to form aself-supporting structure. The polymerizable or curable binder is causedto polymerize by the addition of catalyst and/or heat to convert theformed, uncured foundry sand composition into a hard, solid, curedstate. Examples of curable resin compositions useful as binders in thefoundry art include phenolic and furan resins. In a typical no-bakeprocess, the sand, binder, and a liquid curing catalyst are mixed andcompacted to produce a cured mold and/or core. A binder commonly used inthe no-bake process is a polyurethane binder derived by curing apolyurethane-forming binder with a liquid tertiary amine catalyst.

[0006] Silica sand grains expand upon heating. When the grains are tooclose, the molding sand moves and expands causing a variety of defectsin the castings. One such defect is veining which refers to adiscontinuity on the surface of the casting appearing as a raised,narrow ridge that forms upon cracking of the sand mold or core due toexpansion of the sand during the filling of the mold with the moltenmetal.

[0007] Iron oxides have been used for years in foundry applications toimprove core properties and the quality of castings. Iron oxides haveproven to be advantageous as an additive to foundry molding aggregatescontaining silica sand to improve the quality of castings by reducingthe formation of thermal expansion defects, such as veining, scabs,buckles, and rat tails as well as gas defects, such as pinholes andmetal penetration. There are several iron oxides which are currentlyused in foundries today. These include red iron oxide, also known ashematite (Fe₂O₃), black iron oxide, also known as magnetite (Fe₃O₄) andyellow ochre. Another iron oxide which is presently being used is SierraLeone concentrate which is a hematite ore black in color. Red iron oxideand black iron oxide are the most popular iron oxides in use.

[0008] One method of employing the above iron oxides is to addapproximately 1-3% by weight to the sand mold aggregates during mixing.The exact mechanism by which iron oxides affect surface finish is nottotally understood. However, it is generally believed that the ironoxides increase the hot plasticity of the sand mixture by the formationof a glassy layer between the sand grains which deforms and “gives,”without fracturing at metallurgical temperatures, to prevent fissuresfrom opening up in the sand, which in turn reduces veining.

[0009] Various other types of additives have also been employed in anattempt to improve core properties and the quality of sand castings. Forexample, other anti-veining compounds which have been utilized in sandaggregate mixtures include starch based products, dextrin, fine groundglass particles, red talc and wood flour, i.e. particles of wood coatedwith a resin. All of these additives have met with limited success inreducing veining.

[0010] U.S. Pat. No. 5,911,269 to Brander et al. discloses the use oflithia-containing materials in silica sand molds and cores to reducethermal expansion defects, such as veining. The addition oflithia-containing additives to the foundry sand composition can addsignificantly to the expense of the overall foundry operation.

[0011] Although it is known to use bentonite clays as binders forfoundry green sand molds or cores, bentonite clays have not been used asanti-veining additives in chemically bonded sand compositions. Quitesurprisingly, it has been found that when bentonite clay is used as ananti-veining additive in conjunction with a chemically bonded-basedfoundry sand, the quality of the castings improves by reducing veiningdefects.

[0012] U.S. Pat. No. 4,216,133 to Johnson et al. discloses a shellprocess foundry resin composition containing novolak resinsincorporating from about 0.5 to about 10% based on weight of the resinof a finely divided, siliceous material, such as bentonite. According toJohnson et al., the finely divided siliceous material incorporated inthe foundry resin composition provides peel back resistance andincreased stripping strength. Furthermore, Johnson et al. emphasizedthat the siliceous material is added to the resin material and is notmerely added to the sand mixture in the muller. The incorporation of thesiliceous material is thought to control viscosity during cure. Theamount of siliceous material in the composition based on sand is only0.05 to 0.8%. It should be noted that there is no indication orsuggestion of using bentonite as an anti-veining composition in the '133patent. Veining is not typically considered a problem in a shell moldingprocess.

SUMMARY OF THE INVENTION

[0013] The present invention relates to a method for producingchemically bonded foundry shapes by incorporating an anti-veiningcomposition comprising bentonite into a silica sand aggregate. Theanti-veining composition is mixed with foundry sand used in theproduction of foundry cores and molds to improve the quality of castingsby reducing thermal expansion defects, such as veining, in iron, steeland non-ferrous castings.

[0014] In accordance with one embodiment of the present invention, amethod of producing a silica sand-based foundry shape is disclosed. Theprocess comprises the steps of providing a foundry sand, adding ananti-veining composition to the sand, adding a foundry resin to form asand-based foundry composition, and shaping the sand-based foundrycomposition into a desired pattern, wherein the anti-veining compositioncomprises bentonite.

[0015] The foundry molding and core silica sand mixture used to producecores and molds in accordance with the present invention typicallycomprises about 85% to about 98.5% of commonly used molding and coresilica sand together with about 5 to about 0.5% of foundry resinappropriate for sand cores and molds, and from about 1 to about 10% ofbentonite as an anti-veining additive. The type of bentonite is notparticularly limited and can be a water-soluble sodium bentonite clay ora low-soluble calcium bentonite clay. The composition may also includeother clay minerals such as hectorite, illite, mixtures of illite andthe family of smectites, shale, and other families of clay materials. Inaccordance with particular aspects of the present invention, thebentonite clay may have an average particle size of from about 74μ toabout 3.5 mm.

[0016] The addition of bentonite to foundry molding and corecompositions significantly reduces the casting defects associated withthe thermal expansion of silica and dramatically improves the surfacefinish of such castings. One of the major causes of veining occurs whensilica sand is rapidly heated causing the silica to undergo a rapidexpansion and form fissures that the hot metal penetrates. The additionof bentonite improves the resulting casting quality. Although notwishing to be bound, applicants believe that the reduction in veiningdefects relates to the crystalline structure of bentonite which candecompose and collapse thereby providing room for the expansion of thesilica sand during heating. In addition, it is believed that the loss ofcrystalline water from the mineral reduces gas defects.

[0017] The incorporation of bentonite into the silica sand foundrycomposition substantially improves the surface appearance of the castingand can eliminate or reduce the need for extensive surface grinding toremove any projecting veins from the casting. Accordingly, eliminatingveining can significantly reduce the cost of the casting. Furthermore,bentonite is considerably less expensive than other anti-veiningadditives like lithia containing materials, thereby further reducing thecost of the casting.

[0018] A method for producing foundry shapes by incorporating ananti-veining composition comprising bentonite in the silica sand-basedfoundry composition to reduce veining is disclosed. The processcomprises the steps of providing a foundry sand, adding an anti-veiningcomposition to the sand, and adding a foundry resin to form a sand-basedfoundry composition, and shaping the sand-based aggregate to form adesired pattern, wherein the anti-veining composition comprisesbentonite.

[0019] The bentonite can be selected from the group consisting of sodiumbentonite, calcium bentonite and combinations thereof. The compositionmay include other clay minerals such as hectorite, illite, mixtures ofillite and the family of smectites, shale, and other families of claymaterials. The sand-based foundry composition typically comprises about90% to about 99.5% of commonly used molding and core sand in combinationwith about 5 to about 0.5 of a polymerizable or curable foundry resinappropriate for sand cores and molds, and from about 1 to about 10% ofbentonite.

[0020] The foundry resin typically used is selected from the groupconsisting of phenolic hot box, phenolic urethane, furan, sodiumsilicate including ester and carbon dioxide systems, polyester binders,acrylic binders, alkaline binders, epoxy binders and furan warm boxsystems.

[0021] The bentonite may be utilized in a granular form having anaverage particle size of from about 74μ to about 3.4 mm. Furthermore,the bentonite will typically have a moisture level from about 0.1% toabout 12%, more particularly from about 3 to 5% with a target moisturelevel of about 4%.

[0022] The present invention is also directed to a method of making ametal casting from sand-based foundry compositions comprising the stepsof preparing a sand-based foundry composition of silica sand, a foundryresin appropriate for sand molds, and an anti-veining material, whereinthe anti-veining material is present at a concentration of from about 1to about 10% based on sand, the anti-veining material comprisingbentonite; shaping said sand-based foundry composition to form a sandmold or core having a desired pattern therein; and pouring molten metalinto the pattern formed in the sand-based foundry composition to producea metal casting having little or no veining.

[0023] A foundry core comprising a matrix of sand and foundry resinhaving bentonite particles uniformly dispersed through such that theformation of veins is reduced is also within the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention relates to a method of making a silicasand-based foundry shape wherein an anti-veining composition comprisingbentonite is incorporated in the silica sand-based composition to reduceveining. The anti-veining additive produces a sand-based foundry moldand core composition which resists the formation of some of the defectscommonly associated with the production of castings produced by silica,sand-based molding and core foundry compositions. In particular, theanti-veining additive improves the quality of the castings by reducingthermal expansion defects, such as veining, in iron, steel andnon-ferrous castings.

[0025] The anti-veining additive of the present invention may beutilized with conventional foundry silica sand molding and corecompositions used in the manufacture of sand-based shapes. Such foundrycompositions are typically made from silica sand, with the sand grainsbeing bound together with a mechanical or chemical means. An example ofa commercially available foundry sand is Wedron 520 available fromFairmount Minerals. Typically, the mold or core mixture may comprisebetween about 85% to about 98.5% of silica sand, and about 5% to about0.5% of a foundry resin. The resin used may be of any of numerousconventional core and mold foundry resin systems such as phenolic hotbox, phenolic urethane, furan, sodium silicate including ester andcarbon dioxide system, polyester binders, acrylic binders, alkalinebinders, epoxy binders, and furan warm box systems. A particularlyuseful binder is a no-bake resin binder system available from Ashland.This resin binder system comprises a three part phenolic urethane systemwhich includes a series of binders and a liquid catalyst. Each of theabove binder systems is well known in the art and therefore a detaileddescription thereof is unnecessary.

[0026] The order of additive of bentonite is important to its function.The sand is preferably mixed with the bentonite first and then thefoundry resin is added so that the resin coats the surface of the sandparticles and provides a foundry sand composition with bentoniteparticles dispersed throughout. It is believed that in this manner thebentonite prevents the formation of fissures in the sand.

[0027] The anti-veining composition of the present invention comprisesbentonite. In accordance with one aspect of the present invention,bentonite will be added to the sand-based aggregate in an amount of fromabout 1 to 10% based on sand. More particularly, the bentonite may bepresent in the aggregate in an amount from about 1% to 7% based on sand.Bentonite is a type of clay composed primarily of montmorilloniteminerals. The bentonite used in accordance with the present inventioncan be a sodium bentonite, a calcium bentonite, or mixture thereof. Thecomposition may also contain other materials including clay mineralssuch as hectorite, illite, mixtures of illite and the family ofsmectites, shale, and other families of clay materials.

[0028] In accordance with a particular aspect of the present invention,the bentonite is utilized in a granular form having an average particlesize of from about 74μ to about 3.4 mm. More particularly, the granularbentonite clay may range in size from about 105μ to about 2.0 mm. Theuse of bentonite having a particle size smaller than 74μ has been foundto give rise to no or only very little improvement in casting quality.In particular embodiments of the present invention, the particle size ofthe bentonite is from about 1.0 to 2.0 mm.

[0029] Particles having an average size of about 74μ or greater arethose which are generally retained on the surface of a U.S. standard No.200 mesh sieve screen. Particles having an average size of less thanabout 3.4 mm are those which generally pass through a U.S. standard No.6 mesh sieve screen. Particles having an average particle size of 105μor greater are those which are generally retained on a surface of a U.S.standard No. 140 mesh sieve screen. Particles having a nominal size ofless than about 2.0 mm are those which generally pass through a U.S.standard No. 10 mesh sieve screen.

[0030] The moisture level of the bentonite clay can also affect thequality of the casting. If the moisture level is too high, the productcan potentially fail, and, therefore, it is believed that veiningdecreases with decreasing moisture levels. However, from a practicalstandpoint, the bentonite will typically have a moisture level of from0.1% to about 12%, more particularly from about 3 to 5% with a targetmoisture level of about 4%.

[0031] It is within the scope of the present invention to incorporateother anti-veining compounds in the silica sand-based foundrycomposition. Examples of specific anti-veining compounds include, butare not limited to, dextrin, starch-based products, fine ground glassparticles, red talc, wood flour, and lithia-containing materials. Inaccordance with a particular embodiment of the present invention, theanti-veining composition comprises a mixture of lithia-containingmaterial described in U.S. Pat. No. 5,911,269 and bentonite. The otheranti-veining compounds and more particularly the lithia-containingmaterial can be used with the bentonite in a ratio from about 3 to 1 to1 to 3 lithia-containing material to bentonite.

EXAMPLE 1

[0032] Different silica sand-based foundry compositions were preparedfor the purpose of evaluating various anti-veining additives foreffectiveness in preventing veining and for tensile properties.Accordingly, identical silica sand-based aggregate mixes were preparedutilizing various anti-veining additives. Test samples were prepared byblending the silica sand and the anti-veining material in a mixer for 30seconds. The addition of the 3 part Ashland binder system was completedaccording to the manufacturer's recommendations. The testing specimenswere prepared for evaluation.

[0033] Tables 1 and 2 summarize the effectiveness of variousanti-veining additives. Table 1 is directed to sand cores coated with EZKote Graphite Coating while Table 2 is directed to uncoated cores. TABLE1 Comparison Of Anti-Veining Additives Coated Sand Cores Formula (wt)(g) Example 2 Material Control Example 1 (Comparative) Example 3 Example4 Example 5 Ashland (Part 1) 10 10 10 10 10 10 Pepset XI 1000 Ashland(Part2) 8 8 8 8 8 8 Pepset XII 2000 Ashland Catalyst 3502 0.5 0.5 0.50.5 0.5 0.5 Sand 2000 1900 1900 1900 1940 1860 Bentonite (#40) — 100 —100 60 140 Veinseal — — 100 — — — — Veining (# observed) Horizontal 1None None None 1 None Vertical 2 None None None 2 None

[0034] Examples 1 and 2 illustrate the effectiveness of bentonite as ananti-veining additive as compared to a commercially availablelithium-containing anti-veining additive. Examples 3-5 illustrate theeffect of bentonite concentration on anti-veining. TABLE 2 Comparison OfAnti-Veining Additives Uncoated Sand Cores Formula (wt) (g) MaterialControl Example 6 Ashland (Part 1) Pepset XI 1000 10 10 Ashland (Part 2)Pepset XII 2000 8 8 Ashland Catalyst 3502 0.5 0.5 Sand 2000 1900Bentonite — 100 Veining (# observed) Horizontal 2 None Vertical 4 1(minor)

[0035] The tensile properties of various compositions were calculatedbased on the retained tensile strength in reference to the controlmaterial as indicated in Table 3 below. Tensile strength is important tomaintain the desired form of the mold or core before and during casting.TABLE 3 Comparison Of Anti-Veining Additives Uncoated Sand CoresRetained Tensile Properties of Anti-Veining Aggregates Formulation ofPrepared Mixtures (wt) (g) Example Example Example Example ExampleExample Example Material Control 7 8 9 10 11 12 13 Ashland (Part 1) 1010 10 10 10 10 10 10 Pepset XI 1000 Ashland (Part2) 8 8 8 8 8 8 8 8Pepset XII 2000 Catalyst 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Sand 2000 19701970 1970 1970 1970 1970 1900 Dextrin 30 20 10 — — — — Bentonite (#40) —— 0 0 0 0 0 100 Bentonite (#200) 10 20 7.5 15 22.5 0 Iron Oxide — — — —22.5 15 7.5 — Relative Tensile 100% 34% 7% 1% 39% 7% 4% 47% StrengthVeining (# Observed) Horizontal 3 2 2 2 1 1 1 1 Minor Vertical 4 2 2 0 32 1 0

[0036] Tables 4 and 5 illustrate additional examples in accordance withsome embodiments of the present invention. TABLE 4 Comparison OfAnti-Veining Additives Uncoated Sand Cores Formula (wt) (g) Example 15Material Control Example 14 (Comparative) Example 16 Example 17 Ashland(Part 1) 10 10 10 10 10 Pepset XI 1000 Ashland (Part 2) 8 8 8 8 8 PepsetXII 2000 Ashland Catalyst 3502 0.5 0.5 0.5 0.5 0.5 Sand 2000 1900 19001900 1900 Bentonite (#40) — 100 — — — Veinseal — — 100 — — Calcium Bent.(Gran) 100 Shale 100 Veining (# observed) Horizontal 3 None None None 1Vertical 4 None None None 2 minor

[0037] TABLE 5 Comparison Of Anti-Veining Additives Uncoated Sand CoresFormula (wt) (g) Example 15 Material Control Example 14 (Comparative)Example 18 Example 19 Example 20 Ashland (Part 1) 10 10 10 10 10 10Pepset XI 1000 Ashland (Part 2) 8 8 8 8 8 8 Pepset XII 2000 AshlandCatalyst 0.5 0.5 0.5 0.5 0.5 0.5 3502 Sand 2000 1900 1900 1900 1900 1900Bentonite (#40) — 100 — 25 50 75 Veinseal — — 100 75 50 25 Veining (#observed) Horizontal 3 None None None 1 minor 1 Vertical 4 None NoneNone 1 minor none

[0038] As indicated in the foregoing examples, the addition of bentoniteto molding and core aggregates used in casting manufacture cansignificantly improve quality of the castings by reducing thermalexpansion defects, such as veining. The addition of bentonitesignificantly reduces the casting defects associated with the use offoundry binder systems and molding sand aggregates and increases thestrength the resulting bond aggregates. The use of bentonite as ananti-veining composition reduces the amount of surface grindingnecessary to remove any imperfections at the surface of the casting.Furthermore, the cost of bentonite is less than other anti-veiningadditives thereby providing for lower cost mold and core production,while improving the resulting casting quality.

[0039] Having described the invention in detail by reference toparticular embodiments thereof, it will be apparent that modificationsand variations are possible without departing from the scope of theinvention.

What is claimed is:
 1. A method for producing foundry shapes byincorporating an anti-veining composition comprising bentonite and asilica sand-based foundry composition to reduce veining, said methodcomprising the steps of: providing a foundry sand, adding ananti-veining composition to the sand, and adding a foundry resin to forma sand-based foundry composition, and shaping the sand-based foundrycomposition to form a desired pattern, when the anti-veining compositioncomprises bentonite.
 2. The method of claim 1 wherein said bentonite isselected from the group consisting of sodium bentonite, calciumbentonite and combinations thereof.
 3. The method of claim 2 whereinsaid anti-veining composition further comprises another clay mineralwherein said mineral is selected from the group consisting of hectorite,illite, smectites, shale, and mixtures thereof.
 4. The method of claim 1wherein said sand-based foundry composition comprises from about90-99.5% of a molding and core sanding in combination with about 5 toabout 0.5% of a polymerizable or curable foundry resin appropriate for asand cores and molds, and from about 1 to about 10% of bentonite.
 5. Themethod of claim 4 wherein said foundry resin is selected from the groupconsisting of phenolic hot box, phenolic urethane, furan, sodiumsilicate systems, polyester binders, acrylic binders, alkaline binders,epoxy binders and furan warm box systems.
 6. The method of claim 1wherein said bentonite is utilized in a granular form having an averageparticle size of from about 74μ to about 3.4 mm.
 7. The method of claim6 wherein said bentonite has a moisture level of from about 0.1% to 12%.8. The method of claim 1 wherein said sand-based foundry compositioncomprises from about 85-98.5% of a molding and core silica sand togetherwith about 5 to about 0.5% of a foundry resin appropriate for sand coresand molds, and from about 1 to about 10% of bentonite.
 9. A method ofmaking a metal casting from sand-based foundry compositions comprisingthe steps of: preparing a sand based foundry composition of silica sand,a foundry resin appropriate for sand molds, and an anti-veiningmaterial, wherein the anti-veining material is present in aconcentration of from about 1 to about 10% based on sand and theanti-veining material comprises bentonite; shaping said sand-basedfoundry composition to form a sand mold or core having a desired patterntherein; and poring molten metal into the pattern formed in thesand-based foundry composition to produce the metal casting havinglittle or no veining.
 10. The method of claim 9 wherein said foundrycomposition comprises a matrix of sand and foundry resin havingbentonite particles uniformly dispersed therethrough, wherein saidfoundry composition matrix reduces thermal expansion defects.
 11. Themethod of claim 9 wherein said bentonite is selected from the groupconsisting of sodium bentonite, calcium bentonite and combinationsthereof.
 12. The method of claim 11 wherein said anti-veiningcomposition further comprises another clay mineral wherein said mineralis selected from the group consisting of hectorite, illite, smectites,shale and mixtures thereof.
 13. The method of claim 9 wherein saidsand-based foundry composition comprises from about 90-99.5% of amolding and core sanding in combination with about 5 to about 0.5% of apolymerizable or curable foundry resin appropriate for a sand cores andmolds, and from about 1 to about 10% of bentonite.
 14. The method ofclaim 13 wherein said foundry resin is selected from the groupconsisting of phenolic hot box, phenolic urethane, furan, sodiumsilicate systems, polyester binders, acrylic binders, alkaline binders,epoxy binders and furan warm box systems.
 15. The method of claim 9wherein said bentonite is utilized in a granular form having an averageparticle size of from about 74μ to about 3.4 mm.
 16. The method of claim9 wherein said bentonite has a moisture level of from about 0.1% to 12%.